For future cellular systems, the frequency will be reused in each cell, resulting in inter-cell interference especially for the users residing near the cell edge. There exist several solutions to mitigate this interference based on MIMO beamforming techniques. The gain those solutions are able to deliver may depend on the application scenario as well as on current channel and signal conditions. However, applying the solutions for interference mitigation globally in all the cells imposes a severe increase of the system's complexity. Promising candidates aiming at interference mitigation are cooperative multipoint (CoMP) and interference alignment (IA).
For CoMP, the base stations in a cluster of adjacent cells cooperate and serve the users within the cluster based on joint beamforming techniques. In particular, the antennas at the single base stations are gathered to form a large antenna array. Then, based on the knowledge of the user channels seen from this array, beamforming vectors are calculated that do not cause any interference at the unintended receiving user terminals. While interference is forced to zero by the CoMP technique, it can also deliver a macro-diversity gain through transmitting the useful signal intended for receiving terminal from additional antennas that are distributed in space. According to some implementations, the CoMP approach may require synchronized base stations, synchronized data transmissions from the antennas at different base stations and full knowledge of channels to all users to be served must be available at transmit side. Furthermore, high backhaul requirements may be present, as transmit data may be available at all antenna heads.
In IA, users are still served solely by their assigned base station. A beamforming is used that aligns the direction of the interference from a cell cluster seen at a user terminal, so that the terminal can use its multi-antennas to suppress the interference. The IA technique, however, may require full knowledge of useful and interference channels at the base stations of the cluster. Thus a cooperation between the single sites may not be taken into account, and hence macro-diversity gains may not be realized. According to some implementations, the IA approach may require a knowledge of useful and interference channels to the user terminals to be served; i.e. the channels from all base stations of the cluster seen at the side of the terminal. However, compared to CoMP, the requirements of backhaul and synchronization are rather weak.